1. Field of the Invention
This invention relates to apparatus for interleaving transmitted data. More particularly, the present invention discloses novel apparatus for interleaving and de-interleaving digital data that effectively randomizes channel burst interference.
2. Description of the Prior Art
Radio frequency waveforms employed to transmit communication data have been jammed or interfered with by radio frequency interference (RFI) signals. It is well known that high energy burst of RFI signals or periodic pulsed RFI signals can be made so strong as to completely mask transmitted communication data signals.
It is also well known that interleaving and de-interleaving apparatus can be employed to improve the performance of the transmitted data signals in the presence of burst of RFI signals. When the transmitted data is interleaved, the bits of data may be so distributed over a time period that the RFI signals do not mask or block sequential bits of the original data stream. When the interleaved data bits are de-interleaved at the receiving station, the receive blocks of data appear to have randomized bit errors rather than having broad ommissions of data where burst of RFI signals have occurred. The receive blocks of data bits with bit errors may be corrected by employing a proper type of error correction encoder and decoder that will correct the bit errors.
Row-column interleavers are known where a series stream of data bits are recorded sequentially in a memory array as horizontal rows of data bits. When the data bits are read out of the memory array, they are read out as vertical columns of data bits which effectively reorders the sequence of the series stream of data bits. Convolution interleavers are also known in which a data stream is effectively stored in a shift register and selected bits from different bit positions are read out of the shift register to reorder the stream of data bits. Row-column interleavers and also convolution interleavers are not as effective against periodic burst of RFI signals as would be desired.
Ira Richer has described a block interleaver in the March, 1978, IEEE Transactions on Communications, Vol. 26, No. 3. The described block interleaver effectively takes the block of data bits in a series stream and changes the original serial positions to a predetermined series of new positions similar to the afore-mentioned convolution interleaver. While this block interleaver is described as providing a linear congruential sequence of data bits which perform well in a useful range of periodic RFI duty cycles and repetitive rates it too leaves much to be desired.
It would be desirable to provide an improved or optimized interleaver and de-interleaver for use in a transmitting system subject to periodic burst of RFI timing signals which is effective over an unrestricted range of periodic RFI repetitive rates and jamming duty cycles up to twenty-five percent.